1. Field of the Invention
The present invention relates to an imaging device, an endoscope system including the imaging device, and a method of eliminating noise with the imaging device.
2. Description of the Related Art
Conventionally, for imaging devices having complementary metal-oxide semiconductor (CMOS) image sensors, in order to eliminate fixed pattern noise due to variation of transistors among pixel columns and reset noise of charge-voltage converters within unit pixels, provision of a noise eliminating unit for each pixel column has been known (see, for example, Japanese Patent Application Laid-open No. 2000-59691).
FIG. 15 is a circuit diagram illustrating a configuration of a conventional imaging device. In this example, a case, in which an imaging device 500 has a CMOS image sensor, will be described.
The imaging device 500, for example, is arranged at a distal end portion of an endoscope and includes a light receiving unit and a reading unit. The light receiving unit is configured of: a plurality of unit pixels 530, which are arranged in a two dimensional matrix form over a plurality of rows and a plurality of columns; and a vertical transfer line 539, which transfers a signal output from each of the unit pixels 530. The reading unit is configured of: a vertical scanning unit (row selection circuit) 541; a noise eliminating unit 543, which is provided for each pixel column; and a horizontal scanning unit (column selection circuit) 558.
Each of the unit pixels 530 includes: a photodiode, which accumulates a signal charge according to an amount of incident light; a charge converter, which performs voltage conversion on a signal charge transferred from the photodiode; a transfer transistor, which transfers the signal charge from the photodiode to the charge converter; a reset transistor, which resets the signal charge transferred to the charge converter; a row selection transistor; and an output transistor, which outputs, as an imaging signal, a change in voltage level of the signal charge subjected to the voltage conversion, to a corresponding vertical transfer line 539 in a source follower, when the row selection transistor is in an ON state.
The reading unit turns a row selection transistor of an arbitrary row in the ON state by the vertical scanning unit (row selection circuit) 541 and reads out the imaging signal to the vertical transfer line 539. The read out imaging signal is input to the noise eliminating unit 543 and a noise component thereof is removed. Thereafter, output as image information to outside is performed by the horizontal scanning unit 558.
FIG. 16 is a circuit diagram illustrating a configuration of the noise eliminating unit of the imaging device illustrated in FIG. 15. The noise eliminating unit 543 includes: a transistor 544 for sampling and holding, with one end side thereof connected to the vertical transfer line 539; a coupling condenser (AC coupling capacitor) CC with one end side thereof connected to another end side of the transistor 544; a charge accumulation condenser (sampling capacitor) CS, which is connected between another end side of the AC coupling capacitor CC and ground; and a potential clamp transistor 545, which is connected to a connection node SN between the AC coupling capacitor CC and the sampling capacitor CS. The connection node SN is connected to the horizontal scanning unit 558.
The noise eliminating unit 543 first turns the transistor 544 for sampling and holding to an ON state upon pixel resetting, transmits, by the AC coupling capacitor CC, a noise signal transferred by the vertical transfer line 539, turns the potential clamp transistor 545 to an ON state for a predetermined time period, and samples a noise signal level in the sampling capacitor CS. Thereafter, when the imaging signal is read out, the imaging signal including the noise signal (light-noise sum signal) is transmitted by the AC coupling capacitor CC again. Since a voltage change of the imaging signal after the pixel resetting is transmitted, as a result, the imaging signal from which the noise signal has been subtracted is able to be taken out from the light-noise sum signal.